Here are the Recommended Dietary Allowances (RDAs) for Vitamin B12 according to some US government fact sheet.

There are disagreement s regarding how much B12 we actually need, but almost all experts agree that kids need less B12 than adults. If you find your B12 info from a site that doesn't mention this, this table could be used as a reference for how much you could reduce the B12 intake for children that take B12. One (allergic) side effect B12 could cause is rash, and if you have a baby or child with this or other symptoms that can be associated with a too high B12 intake/allergic reaction, make sure your child doesn't get too much B12.

Birth to 6 months* 0.4 mcg
7-12 months* 0.5 mcg
1-3 years 0.9 mcg
4-8 years 1.2 mcg
9-13 years 1.8 mcg
14+ years 2.4 mcg
Pregnancy 2.6 mcg
Lactation: 2.8 mcg

* Adequate Intake